1. Field of the Invention
This invention generally relates to an optical information recording/reproducing apparatus, and, in particular, to an apparatus for recording or reproducing information on or from an optical disc
2. Description of the Prior Art
A typical prior art optical pick-up for use in an optical information recording/reproducing apparatus, in which information is eraseably recorded on an optical disc, is illustrated in FIG. 4. As shown, the optical pick-up includes a movable optical system 5 which is provided to be movable in the radial direction of an optical disc 1, on which information is eraseably recorded, and which includes an objective lens 2 located opposite to a recording surface of the optical disc 1, a writing coil 3 which is energized when writing information on the optical disc 1, and a light deflecting prism 4. The optical pick-up also includes a stationary optical system 7 which is mounted on a support plate 6 which in turn is fixedly attached to a frame of the optical information recording/reproducing apparatus. In the illustrated example, the stationary optical system 7 includes an illumination optical system 8, a servo optical system 9 and an information (opto-magnetic information) detecting system 10.
In the illumination optical system 8, a laser beam emitted from a semiconductor laser 11 is collimated by a collimator lens 12 and this collimated laser beam is passed through a pair of triangular prisms 13 and 14 to thereby have the laser beam shaped such that it has a circular cross section. This is because, the semiconductor laser typically used as a light source in this type of apparatus emits a light beam which is not circular in cross section but generally elliptic in cross section, having an angle .theta..sub.p of 10.degree. to 20.degree. in terms of half-width angle in the direction parallel to the pn junction of semiconductor laser and an angle .theta..sub.n of 20.degree. to 40.degree. in the direction normal to the pn junction of the semiconductor laser. Thus, such an elliptic laser beam is changed to a circular laser beam by means of the pair of triangular prisms 13 and 14. Then, the laser beam thus reshaped is introduced into a polarization beam splitter 15 for separating incident light from reflected light, which is provided with a polarizing surface 15a for transmitting 100% of p polarized light and reflecting 2/3 of s polarized light. Thus, only the s polarized light component is reflected by the poralizing surface 15a toward the movable optical system 5, so that this s polarized light is deflected by the prism 4 to be focused onto the recording surface of the optical disc 1 through the objective lens 2.
Then, the light reflected from the recording surface of the optical disc 1 is rotated in one (positive) direction or the other (negative) direction around the optical path over a predetermined polarization angle depending on the state of information recorded on the optical disc 1, and due to such rotation, the reflected light beam includes a polarized light component. The reflected light returns to the stationary optical system by following the same optical path and thus it is deflected by the deflection prism 4 to propagate toward the polarization beam splitter 15 and then toward a servo/information detecting light separating polarization beam splitter 16. That portion of the s polarized light component which is reflected by the polarization beam splitter 16 toward the servo detecting system 9 passes through a detection lens 17 and a knife edge prism 18 to impinge upon a tracking light-receiving device 19 and a focusing light-receiving device 20. On the other hand, the light which propagates straight by passing through the polarization beam splitters 15 and 16 includes the remaining portion of the s polarized component and p polarized light enters into the information detecting optical system 10. When this light passes through a half wavelength plate 21 after the polarization beam splitter 16, its plane of polarization is rotated over 45.degree.. Then, the light further passes through a Wollaston polarizing prism 22 to be split into a p polarized light component and a s polarized light component, which are then focused onto a light-receiving device 24 by means of a detection lens 23, whereby the direction of polarization is detected to determine the presence and absence of an information signal.
In the above-described prior art structure, the laser beam emitted from the semiconductor laser 11 is first collimated by the collimating lens 12 and then the thus collimated laser beam is reshaped by the pair of triangular prisms 13 and 14 so as to form a laser beam circular in cross section or circularly symmetrical in light intensity distribution. In this case, as shown in FIG. 5, in order to change the size D.sub.1 of the incident beam in one direction to a desired size D.sub.0 of the output beam in the same direction, two prisms 13 and 14 having the same apex angle are combined with a predetermined angular relationship therebetween. Accordingly, in the illustrated structure, the pair of prisms 13 and 14 must be properly located with respect to the collimator lens 12 and the relative angular relation between the pair of prisms 13 and 14 must be set properly.
As shown in FIGS. 6 and 7, as an alternative structure, use may be made of a pair of cylindrical lenses 25 and 26 for reshaping a laser beam. FIG. 6 shows such a pair of cylindrical lenses when viewed in the direction where no lens action is present and FIG. 7 illustrates such a pair when viewed in the direction where a lens action is present. In this case, use is made of a pair of cylindrical lenses 25 and 26 having focal distances f.sub.1 and f.sub.2, respectively, and they are arranged with their focal points located at the same location on the common optical axis. The focal distances f.sub.1 and f.sub.2 are determined such that the incident light beam having a size D.sub.1 in one direction is converted into an output light beam having a desired size D.sub.0 in the same direction. However, such a structure also requires the provision of two lenses which must be spaced apart over a predetermined distance. Furthermore, the structure shown in FIG. 4 includes a relatively large number of components, and thus it is relatively difficult to manufacture and the overall structure tends to be large in size.